Central core heating in helicon plasmas was simulated in some full wave codes considering classical collision without knowing the clear central heating mechanism since the damping rate of helicon wave was known to be low. However, collisional damping of a fast helicon wave may not be negligible when the fast wave approaches mode conversion surface (MCS) in an inhomogeneous plasma. In this study collisional damping rate of a helicon wave is estimated near MCS from the cold plasma dispersion relation. In the presence of sufficient collision, the WKBJ approximation in inhomogeneous helicon plasmas can be applied even near MCS. The collisional damping rate near MCS is found to be high enough to explain the central heating of helicon plasmas by the helicon wave itself. In this heating mechanism, high density near MCS suffices for the helicon mode and it is closely related to operating parameters such as magnetic field, driving frequency and antenna geometry.